DE3237572A1 - METHOD FOR PRODUCING LAYER IMAGES - Google Patents

Description

-Mr

PHILIPS PATENTVERWALTUNG GMBH PHD 82-111

"Method for generating slice images"

The invention relates to a method of generating of slice images of an object that are generated from several radiation source positions to generate separate individual images is irradiated, with measured values depending on the absorption in its image points being stored for each individual image and for each layer image point an image value corresponding to the absorption from the stored ones Measurement values of the image points of the individual images that are geometrically assigned to the slice image point are derived.

A preferably square area having finite dimensions is used as the image point Designated individual image, while a corresponding area in the layer image is designated with layer image points.

Such a method is known from US Pat. No. 3,499,146. The individual images are created with the help of an X-ray tube generated, which is successively brought into different positions. The radiation relief is from a

Image converter, e.g. an image intensifier, recorded whose 20th

The output fluorescent screen image is scanned with the aid of a television camera. The video signal thus generated represents the sequence of the measured values assigned to the individual pixels of the individual image. It can be in a digital or are stored in an analog memory array. Of the

The image value that is assigned to a slice image point is derived from the image points associated with the slice image point are geometrically assigned, i.e. those image points that were located on the connecting line of the

relevant slice image point with the various 30

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Radiation source positions lie by forming the sum of the measured values. Additional measures can be taken taken to weed out values that deviate significantly from the other measured values.

Go through the described superposition of the measured values assigned to the corresponding image points of the individual images Details that are outside the slice are less strongly reflected in the image of a slice image point than when they are in this layer pixel were located; they are therefore "blurred", as in the other layer recording processes even. In the case of objects with spatially relatively evenly distributed absorption, such a method results useful results. On the other hand, when objects are recorded

Ί5 with spatially very inhomogeneously distributed absorption, E.g. the arteries filled with contrast medium when the coronary arteries are recorded, those outside of a Layer located strongly absorbing details in the layer shown several times, 'and more intensely, depending The lower the number of radiation source positions or the lower the number of individual images. These artifacts impair the evaluation of such layer images considerably.

It is the object of the present invention to design a method of the type mentioned at the outset so that it can be used Layer images of such objects can also be generated with the aid of a few individual images.

This object is achieved in that the Image value is formed by the measured value (s) which corresponds to the lowest absorption (correspond).

The invention is based on the following consideration.

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If an object consisted of only one thin layer, then the absorption or the intensity in all would be one image points of the individual images assigned to certain slice image point are identical to one another and only through the absorption in the Layer pixel determined. In real objects, however, there are usually still on both sides of this layer absorbent structures. As a result, the intensities in the aforementioned image points are lower than in that the former case, and they usually differ from one another. The deviations of the intensities from the former Fall are all the more pronounced, the more the structures absorb outside the layer. In other words: the The highest intensity value has been and is least affected by absorbent structures outside the layer therefore most likely a measure of the absorption in the layer image point. Therefore, if you only use this value as the image value for the layer image point is based on those caused by absorbent structures outside the layer Artifacts in the image of this layer are largely suppressed.

One can therefore with the method according to the invention from only generate four single images (radiation source positions) usable slice recordings.

A further development of the method according to the invention provides before that then the image value assigned to a slice image point from the measured values of this slice image point geometrically assigned individual points is subtracted and that the measured values calculated in this way when generating further Layer images are used as a basis. This development is based on the following consideration: If the measured value that the corresponds to the greatest intensity or the lowest absorption, essentially due to the absorption in the Layer image point is determined, then must also in the measured values of the other image points that make up the layer image point 35

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are assigned geometrically, a corresponding proportion must be available. This portion is obtained through the subtraction eliminated (provided the measured values correspond to the logarithm of the radiation intensity in the respective Pixels). As a result, when generating other slice images of the object, this portion can No longer influencing image values of the slice image points of the other slice, which increases the accuracy.

A device for carrying out the method, which is based on a radiation source arrangement for generating several individual images of the object from one radiation source position each with an image converter arrangement Conversion of each individual image into electrical signals, a memory arrangement for storing the pixels of the Signals assigned to individual images, means for calling up the signals geometrically assigned to a slice image point Signals belonging to pixels from the memory arrangement and for supplying a value derived therefrom to a Image display unit, is characterized in that the signals called up from the memory arrangement are a Extreme value decoder are supplied, the output signal of which forms the signal called up, which is the lowest Absorption. A further development of this device is that the memory arrangement is one of the number of Radiation source positions has the same number of storage areas, each of which corresponds to the pixel of a single image store assigned signal values and which can be addressed simultaneously and independently of one another and that the Image display unit outputs the slice image point by point and is synchronized with the memory area in such a way that in each case the slice image point is reproduced whose geometrically assigned image points are just from the memory area are called. Real-time processing is possible, i.e. the image values of the slice image points are

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just as quickly as they are reproduced by the image display unit, e.g. a television monitor.

The invention is explained in more detail below with reference to the drawing explained. Show it

1 shows a diagram to explain the principle on which the invention is based,

2 shows a device for generating the individual images and to store the measured values that correspond to the pixels that make up the individual images,

assigned,
3 shows a section of the radiation relief produced with the arrangement according to FIG. 2, and FIG. 4 shows a device which determines and displays the image values assigned to the image values of the layer points from the stored measured values.

Fig. 1 shows an object 5 from several radiation source positions or several radiation sources 11, 12, 13 and 14 are irradiated with the same intensity. Among other things, the point or the volume element 6 in a layer 60 shines through. The intensity of the layer image point 6 penetrating and emanating from the radiation sources 11... 14 Radiation is measured by the transducers 71 ... 74 arranged behind the object 5. The radiation-absorbing Structures - e.g. arteries filled with contrast medium - are concentrated in the object in some places, among others in the areas 51, 52 and 53, which are directed to the rays of the radiation sources penetrating the layer image point 6 11, 12 and 13 lie. As a result, the transducers 71, 72 and 73, which are assigned to these radiation sources, measure are, a significantly lower intensity (or a stronger absorption) than the transducer 74, the radiation of the Radiation source 14 detected because the intensity of this

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Converter measured, emitted by the radiation source 14 The beam is practically not influenced by strongly absorbing structures lying outside the layer 60.

If, as with the usual procedures up to now, the Generate image value assigned to slice image point 6 by superimposing the measured values generated by transducers 71 ... 74, then an image value would result which would correspond to a strong absorption in the layer image point 6, even if the actual absorption at this point would be very small. In other words, it would be the structures 51, 52 and 53, which are outside the slice, are projected into the image of the slice. These artifacts will though avoided if only the measured value supplied by the transducer 74, that of the lowest absorption or the highest Intensity corresponds to that which is output as an image value in the slice image point 6. Those outside the shift Structures 51, 52 and 53 then have no influence on the image value of the layer image point 6, i.e. they are not in the shift mapped. This method resides in the apparatus shown in FIG. 1 and in particular in FIG. 4 underlying.

The device according to FIG. 2 contains the four radiation sources 11 ... 14, preferably X-ray tractors, which are arranged in this way are that their focal points are on the corner points of a square. The four radiation sources can be im Operation Emit beams that are illustrated in the drawing by a straight line, but that cover the entire Capture object 5 and be in a plane parallel to the plane of the aforementioned square, which is preferably also the object 5 cuts, practically cover. The radiation sources emit radiation of the same intensity with the same

Radiation duration.
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As can be seen from the explanations of FIG. 1, it is essential for the invention that the individual images are separated i.e. the pixels of a single image may only be generated by one of the four emitters. Basically let this can be achieved by switching on the emitters one after the other (or one emitter one after the other on the corresponding points are brought and switched on), with each frame being saved before the next Single image is made. In this case, the object 5 would be irradiated at different points in time, which in itself moving or moving objects could lead to blurring. It would also be possible to change the transducer arrangement that supports the converts spatial radiation intensity patterns into electrical measured values, to be arranged so far away from object 5, that the radiation reliefs created by the four emitters side by side on the input surface of the transducer array Find place. In this case, however, the radiation dose would have to be increased and the input area would have to be correspondingly be great.

To generate separate - albeit nested - individual images, the DE-PS 25 46 785 known, a two-dimensional grid 8 used, which in a to the plane of the square mentioned parallel plane and which has as many square openings as the individual image has pixels should, e.g. 256 χ 256 (or 512 χ 512). The grid 8 is arranged between the object 5 and a transducer arrangement 7, that four pixels are irradiated through each opening by the four radiators 11 ... 14, wherein these four pixels are adjacent to (and not overlap with) other pixels that are covered by a adjacent opening are irradiated.

The result is therefore on the input surface of the image converter 7

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an image, a section of which is shown in FIG. 3. In this case, 1, 2, 3 and 4 pixels are designated, the from the radiators 11, 12, 13 or 14 through the various Openings in the grid 8 are generated through it. How one recognizes from Fig. 3, the pixels of every second line are generated alternately by the radiators 11 and 12, while the pixels of the lines lying in between are generated by the radiators 13 and 14.

The image converter can basically be represented by a matrix of Semiconductor detectors are formed whose dimensions correspond to the dimensions of the pixels and their number is four times as large as the number of pixels in a single image, so that each of the four radiation sources generated pixels detected by a separate detector could be. However, such a detector arrangement is still relatively expensive. Therefore, an image intensifier is used in its place 7 is used, on the input screen of which the intensity reliefs corresponding to the individual images are separated,

however, interleaved according to FIG. 3, impinge. At the At the output of the image intensifier, the input image appears reduced, but with much greater intensity. This initial image is recorded with the aid of a television camera 9. If the line direction, with that in the TV camera 9 the image on the output screen of the image intensifier 7 is scanned, in the direction of arrow 70 in FIG Fig. 3 is placed, and when the number of lines of this television camera is twice as large as the number of lines of pixels in a single image (with 256 χ 256 pixels per single image so 512 lines; with 512 χ 512 pixels 1024 lines), one of the rows of pixels visible in detail from FIG. 3 is scanned with each line.

The video signal generated by the television camera is scanned twice as often as pixels in a line

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the rows of a single image are available, i.e. 512 or 1024 times in the examples given, and with the help of an analog-to-digital converter 10 converted into a binary number or digital data word, which is thus a measure of the Represents the intensity or the absorption in the relevant image point. Appropriately, the video signal is before the Digitization logarithmized or its dynamic range restricted by a suitable compander, whereby on it Care must be taken that the binary numbers are reversible and clearly reflect the absorption or radiation intensity in the remain assigned to individual pixels.

The arrangement comprises four digital memories 21 ... 24, of which each is assigned to one of the emitters 11 ... 14, i.e. each this memory stores the data words which the associated radiator 11 ... 1.4 generated pixels assigned are. For this purpose, the output line of the analog-to-digital converter (actually a data bus) is via a The first demultiplexer 16 shown as a changeover switch is connected to the inputs of two further demultiplexers 17 and 18. The outputs of the demultiplexer 17 are connected to the data inputs of the memory 21 and 23, while the Outputs of the demultiplexer 18 with the data inputs of the

Memories 22 and 24 are connected.
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A clock generator 15, which also generates the synchronizing signals for the television camera 9, switches the demultiplexer 16 in same clock in which the analog-to-digital converter under the influence of a likewise from the clock generator 15 generated clock signal data words. As a result, those generated by the analog-to-digital converter 10 are generated Data words alternately supplied to the memory 21 and the memory 22 via the demultiplexer 16, as long as the Demultiplexers 17 and 18 have their switching position shown in the drawing. If the data words generated thereby

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the absorption or the intensity in pixel row 1, 2,1,2 etc. (see. Fig. 3) correspond to this Thus, the data words assigned to the pixels 1 (FIG. 3) in the memory 21 and the data words assigned to the pixels 2 Data words are taken over into memory 22. The addressing of this memory is carried out by the clock generator 15 with the half the clock frequency switched over like the analog-to-digital converter 10.

At (or immediately before) the beginning of the next TV line the demultiplexer 17 and 18 are switched to the switching position, not shown in Fig. 2, so that then the Data words which are assigned to the pixels 3 and 4 in FIG. 3 are read into the memories 23 and 24 will. In the next line, the demultiplexers 17 and 18 are again in the switching position shown returned, etc. When the television camera 9 converts the entire output screen of the image intensifier 7 into a video signal has implemented, in each of the memories 21 ... 24 is the intensity or absorption distribution of the four X-ray tubes 11 ... 14 generated individual images are stored.

To generate an image of that layer in which the emitted by the four X-ray emitters 11 ... 14 Covering the bundle of rays, each of the memories 21, 22, 23 and 24 becomes the first data word stored therein read out and fed to the four inputs of an extreme value decoder 19 via corresponding bus lines. This switches the data word at its four inputs that

³ ^ corresponds to the largest binary value and therefore the greatest intensity, through to its single output, to which a digital-to-analog converter 25 is connected, which is connected to a television monitor 27 via a switch arrangement 26. Four more data words are then read out simultaneously from the memories 21 ... 24, in each case in the order in which they are stored in the relevant memory

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have been read. The clock generator 150 supplies the address counters 210, 220, 230 and 240 with the memories 21 ... 24 are assigned, clock pulses and also the deflection generators of the Pernsehmonitor 27, so that these are synchronized in such a way that the converted into analog signals at the video input of the television monitor 27 Words of data appear at the rate required to produce a television picture representing the layer are needed.

If images of other layers are to be generated, the data words that correspond to the pixels must be called up belong, which are geometrically assigned to the respective slice image point to be generated. These data words are in memories 21 ... 24 are no longer stored at the same address. When calling up the data words, the Addresses must therefore be modified accordingly.

If single images with e.g. 256 χ 256 pixels are processed 20

this modification of the addresses is relatively easy if it is assumed that the location of a Pixel within a pixel row or row is determined by the last eight binary digits of the address and that the position of a row of pixels within a single image by the first eight binary digits of the Address word is determined. The last eight binary digits of the memories 21 and 24, which the emitters 11 and 14 are assigned whose connecting line is perpendicular to the direction of the row of pixels must be around the

the same amount (e.g. + x) can be changed. The same applies to the change of the last eight binary digits of the address words for the memories 22 and 23, but this change must be the the former be opposite to each other (-x). The first eight binary digits of the address words for the memories 21 and

22 must be changed by the same amount (+ y). That-

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The same applies to the memories 23, 24, but with the opposite Sign (-y).

The required address offset can easily be implemented if the address counters 210 ... 240 are preset Binary counters are designed to which the preset value (x, y) is specified in the manner described. This can be done with the help of a keyboard or a analog input device operable layer selection device 27 take place. In this way it is possible to get out to subsequently generate images of any slices for the individual images, the position of which is clearly defined by the parameters x, y is determined.

The extreme value decoder 19 will now be described in more detail. It receives four bus drivers 31 ... 34, each of their inputs are connected to the data outputs of the memory 21 ... 24 assigned to them via a bus line. The input signal of such a bus driver 31 ... 34 appears (amplified) at its output if a corresponding signal is present at its price input. The release inputs of the Bus line drivers 31 and 32 are controlled by a comparator 35, the inputs of which via bus lines with the Outputs of the memory 21 and 22 are connected. Of the

The comparator has two outputs that signal whether the am Output of memory 21 or the binary number present at the output of memory 22 is greater. A third exit 36 indicates that both binary numbers are equal. The three outputs of the comparator are direct or via one

Or link with the enable inputs of the bus line driver 31 and 32 connected so that the bus line driver 31 is enabled when the memory 21 supplied Binary number is greater than the binary number at the output of the memory 22, and that the bus line driver 32 enables

will be given when the binary number at the output of the memory 22

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is equal to or greater than the binary number at the output of the Memory 21. The bus line driver .31 and 32 is therefore located at the two interconnected outputs the larger of the two binary numbers.

In the same way, the bus line drivers 33 and 34 are controlled by a comparator 37 so that they are connected to each other connected outputs the larger of the two binary numbers appears or - if both binary numbers are equal - this binary number. The output 38 of the comparator 37 signals that both binary numbers are the same are.

The interconnected outputs of the bus conductors 31, on the one hand and the bus line driver 33 and 34 on the other hand are connected to the inputs of a third comparator 45 as well as with a further bus line driver or 47, which in turn are controlled by the comparator 45 be that at their outputs connected to one another and to the input of the digital-to-analog converter 25, in each case the larger of the two binary numbers appears or - if the two binary numbers are equal to each other - one of these Binary numbers. In this way, the digital-to-analog converter 25 receives the largest of the four memories at the outputs 21 ... 24 applied binary numbers or, if several binary numbers have the maximum value, one of these binary numbers.

It is clear that the extreme value decoder 19 at his Output must deliver the smallest binary number in each case if the video signal in the memories 21 ... 24 is in such a form it is stored that the smallest absorption or the greatest intensity is assigned the smallest binary numbers are. In this case, only the release inputs of the two bus line drivers controlled by a comparator need (e.g. 31, 32) can be swapped with each other.

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If there is an intensity in three of the four image points geometrically assigned to a layer image point (e.g. 6 in FIG. 1) is measured, which is significantly smaller than the intensity in the fourth pixel, then this can have two causes:

a) Three radiation sources shine through apart from the slice image point 6 still further absorbent structures (51 ... 53 in Fig. 1) which are outside the layer.

b) There is a measurement error.
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So this means that in the event that there is only one maximum value is found, there is always a certain uncertainty as to whether this measured value is actually due to the absorption ratios is determined in the slice image point or by a measurement error.

If, on the other hand, two maximum values of at least approximately the same size measurement errors are less likely. This is even more pronounced when three or even four are close to equally large maximum values are present. This means that the slice image is composed of image values for which the Probability that the relevant value has been measured correctly, from slice pixel to slice pixel can fluctuate.

Layer image points to which only one (or two or three) maximum value (s) can be assigned and which are therefore more uncertain than slice image points to which more maximum values can be assigned, can be blanked with the help of the switch 26, the video input of the television monitor 27 always with a suitable voltage that can be tapped off e.g. at a potentiometer 55 (instead of with the Output of the digital-to-analog converter) connects if less than two (or three or four) maximum values are present are. The switch 26 is controlled by a logic circuit 56, the signals on lines 36, 38 and the corresponding line 48 of the comparators 35, 37, 45

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and one of the other two output lines of the comparator 45 linked to one another. The signal state on these four lines clearly define whether one, two, there are three or four maximum values. The logic combination circuit 56 is expediently designed in such a way that that the user can specify through a suitable input device how many of the four of the Saving the binary numbers supplied must correspond to the maximum value. Since, as already explained at the beginning, all that matters is that the binary numbers at least approximately correspond to the maximum value, need the comparators 35, 37 and 45 not all binary digits of a binary number are fed, but only the most significant.

In this way it is possible for the user to specify the degree of security for the individual layer image points.

From the above it follows that from the four recorded by simultaneously switching on the X-ray tracers 11 ... 14 Individual images layer images can be produced, the position of which can be predetermined and in which the Security that layer image points are not based on measurement errors can be specified. The length of time required to generate this layer image corresponds to the duration of two television images and the switch-on time. It is possible to switch on the four X-ray tractors periodically in order to record dynamic processes and from them Making layer images under fluoroscopy ("tomoscopy") · If, in addition, the television signals of the series of images generated in the process are stored on a suitable memory (e.g. magnetic tape) are saved, subsequent processing is possible. It is also possible to save the individual images that are generated one after the other by e.g. two X-ray tubes lying next to one another, as images of a stereo video film to process. Each projection direction can be used as a

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Video film. Of course, each picture can be shown as a freeze frame.

The described layering method is particularly suitable for images such as those used in digital subtraction angiography attack. In order to carry out this subtraction in the time or energy mode, the hardware described from FIG. 4 to be used. For recording in energy mode, the tubes must be operated one after the other with different voltages will.

It is also conceivable that this method can be used with film (conceivable is also Futji Platte). It is particularly advantageous that the image carrier is planar and consequently none Image geometry errors occur. The film images can be digitized with known film scanners (Optronix Photoscan) will. However, a lens matrix (lenses arranged like X-ray tubes), which contains the various perspective images, is also suitable one after the other - via photo shutters - projected onto a television camera. The further processing then takes place according to Fig. 2. (lens matrix for decoding layers is described in the German patent application P 27 46 035.1). In principle, the primary images can be post-processed before they are further processed, for example, into layer images

known methods are preprocessed (e.g. high-pass filtering, differentiation, etc.). In addition to the receiving tubes (4), a tube can be provided in the middle, for example for positioning the patient and for catheterizing vessels.
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Claims (9)

PHD 82-111 PATENT CLAIMS: (Ty Method for generating slice images of an object that is irradiated from several radiation source positions to generate separate individual images, with measured values that are dependent on the absorption in its image points being stored for each individual image and for each layer image point an image value corresponding to the absorption from the stored measurement values of the Slice image point is derived geometrically associated image points, characterized in that the image value is formed by the measured value (s) which corresponds to the lowest absorption. 2. The method according to claim 1, characterized in that the image value assigned to a slice image point is then subtracted from the measured values of the individual image points geometrically assigned to this slice image point and in that the measured values calculated in this way are used as a basis for the generation of further slice images. 3. The method according to claim 1, characterized in that for each slice image point the number of those geometrically assigned individual image points is determined, the measured values of which correspond at least approximately to the maximum value of the absorption. 4. Device with a radiation source arrangement (11 ... 14) for generating several individual images of the object (5) from one radiation source position each with an image "Converter arrangement (7, 9) for converting each individual image ο /.ο ι ϋ / c -> 8 ~ - PHD 82-11 1 -JL- into electrical signals, a memory arrangement (21 ... 24) for storing the signals assigned to the pixels of the individual images, with means (28; 210 ... 240; 150) for calling up the signals belonging to the pixels geometrically assigned to a slice pixel from the Memory arrangement (21 ... 24) and for supplying a value derived therefrom to an image display unit (27), characterized in that the signals called up from the memory arrangement (21 ... 24) are supplied to the image display unit via an extreme value decoder (19), whose output signal forms the signal called up which corresponds to the lowest absorption. 5. Apparatus according to claim 4, characterized in that the memory arrangement has a number of memory areas (21 ... 24) equal to the number of radiation source positions, each of which stores the signal values assigned to the pixels of a single image and which can be addressed simultaneously and independently of one another and that the image display unit (27) has the Outputs slice image point by point and is synchronized with the memory areas in such a way that the slice image point is reproduced in each case, the geometrically associated image points of which have just been called up from the memory areas. 6. Apparatus according to claim 4, characterized in that an X-ray tracer (21 ... 24) is arranged in each radiation source position and that the X-ray tracers can be switched on simultaneously or one after the other. 7. Apparatus according to claim 6, characterized in that a single image converter is provided and that between the X-ray emitters and the image converter (7, 9) a grid (8) with one of the number of BAD ORIGSNAL - Υ9 - PHD 82-111 -3- The number of openings corresponding to pixels of an individual image is arranged in such a way that each point (1, 2, 3, 4) on the image converter is irradiated only by one X-ray beam.
5 8. Device according to one of the preceding claims, characterized / that four radiation sources are provided which are arranged on the corner points of a square are.
10 9. Apparatus according to claim 4 and claim 8, characterized in that the extreme value decoder contains four circuits (31 ... 34) with gate effect, which are coupled to the outputs of four memories (21 ... 24) in which the Measured values assigned to pixels of the individual images are stored, and that a comparator (35 or 37) is provided for every two of these circuits (31 and 32 or 33 and 34), which compares the input signals of these circuits with one another and controls the circuits accordingly the outputs of these two circuits are connected to one another and to the inputs of two further circuits (46, 47) with gate effect, which are controlled by a further comparator (45) which compares the input signals of these circuits, and their interconnected outputs the output of the extreme value decoder represent.